Efficacy of human leukocyte interferon in the prevention of CMV infection, relapse of leukemia, and graft-versus-host disease after marrow transplant for acute lymphocytic leukemia in remission is being studied in a randomized protocol. Initial studies have identified a dose that seems both to be safe and to have measurable biological effects when given every 3 days to marrow transplant patients. Preliminary analysis suggests, for example, that the natural killer (NK) activity of circulating lymphocytes is increased in treated patients. Data on the clinical efficacy of interferon should be forthcoming in the next 1-2 years.